Rubber is normally reinforced with selected reinforcing fillers to enhance its physical properties, particularly stiffness and modulus.
Particulate reinforcement such as, for example, carbon black is often used to enhance the modulus of rubber vulcanizates.
Various discontinuous (non-woven) fibers have also been used, particularly to enhance both stiffness and modulus of rubber vulcanizates. For example, discontinuous cellulose fibers have been used as dispersions thereof in rubber as disclosed for example in U.S. Pat. Nos. 3,697,364, 3,802,478, and 4,236,563. Other discontinuous fibers have been suggested or used such as, for example, polyamide (nylon), cotton, rayon, polyester. glass and steel.
It has sometimes been taught that, for good and effective elastomer reinforcement, a high aspect ratio, or length to diameter (L/D) ratio of the fiber was important. Thus, for typically described fibers with a rod-like, needle-like or thread-like structure, it was first desired that they be very short and also to have very small diameter, in order to achieve, for example, an aspect ratio of about 50 or higher; see for example U.S. Pat. No. 3,697,364 previously cited.
Normally, where discontinuous fibers are used, the rubber composite also contains a particulate reinforcement in combination therewith.
Aramid in the form of strands of woven yarns, is well known as a fibrous reinforcing agent. Various grades are available, e.g. "Kevlar", "Kevlar" 29, and "Kevlar" 49, all available from E. I. Du Pont de Nemours & Co., Inc., Wilmington, Del. "Kevlar" is available as continuous filament yarn, short fiber and pulp according to the manufacturer. Cords of stranded "Kevlar" are used in the belts of radial car tires and carcasses of radial truck tires. "Kevlar" 29 is available in staple length and as a pulp, and, according to "Modern Plastics Encyclopedia", 1984-1985, page 125, is a low density, high strength aramid fiber used in ropes, cable, coated fabrics, friction products such as brakes and clutches, and in gaskets. "Kevlar" 49 according to "Modern Plastics Encyclopedia" is available either as a yarn or a pulp, and is used as a reinforcement for plastic composites, e.g. in aircraft, aerospace, marine and automotive applications, and in sports equipment. Aramid in any of its forms is essentially inextensible.
U.S. Pat. No. 4,155,394 describes cord composites for rubber tire reinforcement comprising a plurality of plies of different yarns, e.g. nylon and aramid, which are helically twisted to form a cable.